Mobile communication systems were developed to provide the subscribers with voice communication services on the move. With the rapid advance of technologies, the mobile communication systems have evolved to support high speed data communication services beyond the early voice-oriented services. However, the limited resource and user requirements for higher speed services in the current mobile communication system spur the evolution to more advanced mobile communication systems.
As one of the next-generation mobile communication systems to meet such requirements, standardization for a Long Term Evolution (LTE) system is underway in the 3rd Generation Partnership Project (3GPP). LTE is a technology designed to provide high speed packet-based communication of up to 100 Mbps and aims at commercial deployment around 2010 timeframe. In order to accomplish the aim, a discussion is being held on several schemes: one scheme for reducing the number of nodes located in a communication path by simplifying a configuration of the network and another scheme for maximally approximating wireless protocols to wireless channels.
FIG. 1 illustrates a typical LTE mobile communication system architecture.
Referring to FIG. 1, the radio access network of the LTE mobile communication system includes next generation base stations (hereinafter, referred to interchangeably as Evolved Node B (eNB), UTRAN, and Node B) 105, 110, 115, and 120, a Mobility Management Entity (MME) 125, and a Serving-Gateway (S-GW) 130.
The UE 135 connects to an external network via eNBs 105, 110, 115, and 120 and the S-GW 130, and Packet Data Network (PDN) Gateway (P-GW) (not shown).
The eNB is a Radio Access Network (RAN) node corresponding to the Radio Network Controller (RNC) of the Universal Terrestrial Radio Access Network (UTRAN) and the base station controller of the GSM EDGE Radio Access Network (GERAN). The eNBs 105, 110 115, and 120 are connected to the UE 100 through a radio channel and responsible for the role of legacy RNC/BSC. An eNB can serve several cells.
In the LTE system, all the user traffic including real time services such as Voice over Internet Protocol (VoIP) is served through a shared channel and thus there is a need of a device capable of collecting status information of the UEs and scheduling the UEs. The eNB is responsible for collecting the status information of the UEs and scheduling the UEs based thereon.
The S-GW 130 is an entity responsible for providing data bearer and establishes or releases the data bearer under the control of the MME 120.
Although not depicted, the core network of the LTE mobile communication system further includes an Application Function (AF), a Policy and Charging Rules Function (PCRF), and a P-GW in addition to the above-described MME 120, and S-GW 130.
The AF is an entity responsible for exchanging information related to application with the user on the application level.
The PCRF is an entity for controlling the policy related to Quality of Service (QoS) and transfers the Policy and Charging Control (PCC) rule to the P-GW. The PCRF is an entity of controlling the QoS and billing for the traffic.
Meanwhile, the term “UP” denotes the paths connecting between the UE 100 and one of the RAN nodes 105, 110, 115, and 120; between the RAN node and the S-GW 130; and between the S-GW 130 and the P-GW (not shown). Among them, the path between the UE 100 and the RAN node is the most resource-restrictive radio channel.
In the radio communication system such as LTE, QoS is applied per Evolved Packet System (EPS) bearer. An EPS bearer is used to transmit the IP flows requiring the same QoS. The EPS bearer is designated parameters related to QoS such as QoS Class Identifier (QCI) and Allocation and Retention Priority (ARP). The QCI is a parameter defined as an integer indicating QoS priority, and the ARP is a parameter for use in determining whether to accept or reject new EPS bearer establishment.
The EPS bearer corresponds to the Packet Data Protocol (PDP) context of the General Packet Radio Service (GPRS). An EPS bearer belongs to a PDN connection which has the Access Point Name) as an attribute. When a PDN connection for IP multimedia Subsystem (IMS) service such as Voice over LTE (VoLTE), the corresponding PDN connection is established using the well-known IMS APN.
In order to support voice telephony in the LTE network, the Packet Switched (PS) mode IMS-based VoLTE or the Circuit Switched Fall Back (CSFB) reuses the Circuit Switched (CS) mode of the 2nd Generation or 3rd Generation (2G/3G) system. VoLTE is the term used in the same concept as Voice over IMS (VoIMS). Typically, the 2G system denotes Global System for Mobile Communications (GSM) or the system using the GSM EDGE Radio Access Network (GERAN) as the Radio Access Network (RAN), and the 3G system denotes the Universal Mobile Telecommunications System (UMTS) or the system using the Universal Terrestrial Radio Access Network (UTRAN) and the RAN.
In the radio communication system, particularly the LTE system, if an inbound or outbound voice call occurs if the UE is connected to the LTE network, the CSFB procedure is executed for switching to the CS network. In this case, a UE authentication procedure is performed so as to cause voice telephony service delay. Typically, the 2G/3G system is a CS network capable of providing CS service, and the CS service-related control entity is the Mobile Switching Center (MSC) or Visitor Location Register (VLR). The CSFB responsible for the function of switching to the CS service is performed using the SG's interface between the MSC/VLR and MME.